Despite massive international efforts to control Schistosomiasis, there is yet no effective control method for either the parasite or the snail and the number of infected people has continued to increase to the currently estimated 200 million. A few reports of the incidental infection of schistosomatoid larvae in their snail hosts by microsporidans suggest the possibility that microsporidosis in trematodes may be employed as a control method. We, therefore, propose to evaluate the potential of microsporidans as biological control agents for Schistosoma mansoni in its snail host, Biomphalaria glabrata. If successful, this research could lead to the effective control of S. mansoni with a specific, self-perpetuating agent which does not leave a toxic residue.